This invention relates to a rack-and-pinion type steering gear particularly for use with an automotive vehicle.
In a conventional rack-and-pinion type steering gear as shown in FIGS. 1 and 2, a pinion shaft 4 having a pinion 3 at its one end is rotatably supported by bearings to a rack housing 2 fixed to an automotive body and a rack bar 5 having rack teeth 6 meshed with the pinion 3 is inserted into an insert hole 13 formed in the rack housing 2. The other end of the pinion shaft 4 is connected to a steering main shaft (not shown) and both ends of the rack bar are connected through tie rods 20 to knuckle arms (not shown) of an axle shaft. The rack housing 2 is formed with a rack guide hole 14 on the opposite side to the meshed position of the pinion 3 and the rack teeth 6, and a rack guide 7 is inserted into the rack guide hole 14. The rack guide 7 is formed with a semi-circular guide surface 11 at its front end which surface is abutted against the back surface of the rack bar 5. A set bolt 8 is screwed into the rack guide hole 14 from the lower side of the rack guide 7 and serves to restrict a regressive stroke of the rack guide 7. A compression spring 9 is interposed between the set bolt 8 and the rack guide 7 and normally biases the rack guide 7 toward the rack bar 5. There is defined a guide clearance t' between the lowermost end of the rack guide 7 and the uppermost end of the set bolt 8 so as to allow the rack guide 7 to regress along the rack guide hole 14 by a given length. (See FIG. 2.)
In relation with the above-mentioned arrangement, when a large torque of the pinion 3 is transmitted to the rack teeth 6, there is created a repulsive force acting to repulse the rack bar 5 from the pinion 3 corresponding to the pressure angle of the pinion 3. Such a downward movement of the rack bar 5 due to the repulsive force is restrained within its permissible limit by the action that the rack guide 7 fills the guide clearance t' and abuts against the set bolt 8. Accordingly, the rack guide 7 is generally made of a high-strength material such as a metal so as not to be damaged even if a large repulsive force is created during a rough road running of a vehicle and the like. However, in the rough road running, a repulsive force greater than the biasing force of the compression spring 9 is likely to be frequently created and every time of the creation of the repulsive force, the rack guide 7 collides with the set bolt 8, thereby disadvantageously causing a creation of noise.
As measures against the above drawbacks, it is effective to reduce a rigidity of the rack guide 7 and to this end, it is conceivable to make the rack guide 7 of a low-rigid material such as a synthetic resin as well as improve the shape of the same. This is desirable from a viewpoint of lightening the weight of a vehicle and reducing the cost for manufacturing the steering gear. However, the rack guide made of resin is hard to obtain a sufficient strength so as not to be damaged upon receipt of a large repulsive force.